Traditionally, many exposed automotive components are chrome-plated to provide an anti-corrosion coating and an aesthetic quality to the underlying steel component. Based upon the details of the chromium coating process, a steel component accepted a yellow or black hue or took on a silver-mirrored appearance.
In recent years, the toxic and environmental degradation associated with the handling and disposal of chromium and its precursors has become known. In particular, hexavalent chromium salts are recognized to be particularly deleterious. In spite of the need to eliminate chromium from automotive steel plating, a desire remains to have the cosmetic qualities of a chromium yellow and chromium black finish. While it is well known that acidic molybdenum salt solutions can plate a black surface layer onto steels having a zinc, zinc-nickel, or zinc-chloride treated surface, formation of a suitable surface finish yellow coating has proven considerably more difficult.
Thus, there exists a need for a process that provides a yellow hued coating on zinc treated steel that is chromium free.